1. Field of the Invention
The present invention relates to a heat pump drying machine which is provided with an accommodation room for accommodating a drying target and is configured to execute a dry operation for the drying target in the accommodation room.
2. Related Art
The heat pump drying machine is generally known to have the following configuration. The heat pump drying machine is provided with a rotary drum and a heat pump. The rotary drum accommodates a drying target. The heat pump is provided with a refrigeration circuit comprising a compressor, a radiator, an expansion device, an evaporator and the like. The heat pump drying machine is configured to operate the compressor at a predetermined operation frequency and make refrigerant discharged from the compressor flow through a radiator, the expansion unit and the evaporator. Simultaneously, the heat pump drying machine is configured to dry the drying target in the rotary drum by circulating air in an air blowing path from the radiator to the evaporator via the rotary drum (see e.g., JP-A-2008-086693 and 2006-075217).
By the way, the heat pump drying machine of this type is required to be able to execute an energy saving dry operation with an enhanced COP (coefficient of performance) in addition to a normal dry operation in which the compressor is operated at the above predetermined operation frequency. In the energy saving dry operation, power consumption is further reduced than that in the normal dry operation although a drying time is longer than that in the normal dry operation. When the energy saving dry operation of this type is executed, however, it takes a longer time as compared with the normal dry operation until the drying target is warmed and can be used as a heat absorption source in a heat pump cycle. During this period, such a condition that pressure of the compressor cannot be easily increased occurs.
Especially, when the external temperature is low in some conditions (e.g., winter) and the atmospheric temperature of the heat pump drying machine is also low, temperature of the air circulating in the air blowing path is accordingly lowered. In order to execute heat exchange of refrigerant with the circulating air, the refrigerant cannot absorb heat (i.e., energy) from the circulating air without controlling the temperature of the refrigerant flowing through the evaporator to be lower than that of the circulating air.
Therefore, the temperature of the refrigerant flowing through the evaporator may be equal to or less than 0° C. until the temperature of the circulating air is increased to a fixed temperature or more. In this case, condensed water generated in the evaporator is frozen there, and the frozen condensed water may block the air blowing path. Consequently, there occurs such a drawback that the air cannot be circulated in both of the accommodation room and the air blowing path, so that the drying efficiency is lowered.
To solve the drawback, the aforementioned conventional technique discloses a configuration that a supercooling pipe is provided to the evaporator to supercool the refrigerant discharged from the radiator. Accordingly, the evaporator is defrosted by means of heat of the refrigerant that flows through the supercooling pipe. In the configuration, however, the heat transfer area of the evaporator is reduced by the amount corresponding to the supercooling pipe. Therefore, there is a problem that operation efficiency in the normal dry operation is lowered.
Furthermore, there is also a problem that the control is cumbersome to implement energy saving without deteriorating an actual drying function.